Characterizing a Time–Memory Tradeoff Against PudgyTurtle

Abstract

PudgyTurtle is not a cipher, but rather an alternative way to utilize the keystream in binary-additive stream-cipher cryptosystems. Instead of modulo-2 adding the keystream to the plaintext, PudgyTurtle uses the keystream to encode 4-bit groups of plaintext, and then to encipher each codeword. One goal of PudgyTurtle is to make time–memory tradeoff attacks more difficult. Here, we investigate one such attack (a modification of the well-known Babbage–Golić method), and show that its time-complexity is harder on average than an analogous tradeoff attack against a standard binary-additive stream cipher; may approach that of a ’brute-force’ attack; can be reduced by certain parameter choices; and can be formulated in terms of a probability distribution which is amenable to simulation.